Crystal structure refinements of disordered pb(sc_0.5nb_0.5)o₃ in the paraelectric and ferroelectric states

Abstract

The crystal structure of the relaxor ferroelectric Pb(Sc₀.₅Nb_0.5)O₃ has been refined from high-resolution neutron time-of-flight powder diffraction data in the paraelectric and ferroelectric states. In the ceramic used in this study, no evidence was found for unit cell doubling, associated with B-site cation ordering, consistent with the (NH₄)₃FeF₆ structure in the paraelectric state. At 400 K, in the paraelectric state, Pb(Sc_0.5Nb_0.5)O₃ is cubic, space group Pm3m, with lattice parameter 4.08153(1) Å, and at 200 K, in the ferroelectric state, it is rhombohedral, space group Ram, with lattice parameters a = 4.08207(1) Å and a = 89.872(1)°. Calculated absolute displacements in the ferroelectric state along the [111] ferroelectric axis are for Pb 0.094 &, Sc/Nb -0.148 Å. and O -0.195 Å. The 3.0 Å coordination shell around the Pb cation shows evidence for a stereochemically active lone pair, whilst by contrast, the distortion in the SclNb octahedral site is small.

Key Words:

• Relaxor ferroelectrics
• crystal structure
• neutron diffraction
• powder diffraction.